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Evaluating Water Quality. Abiotic and Biotic Factors. What is water quality?. Appearance of the water Ability for humans to drink and use the water Clean enough for organisms that live in the water. Abiotic Factors. Certain abiotic factors affect water quality. For example:

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Evaluating Water Quality

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evaluating water quality

Evaluating Water Quality

Abiotic and Biotic Factors

what is water quality
What is water quality?
  • Appearance of the water
  • Ability for humans to drink and use the water
  • Clean enough for organisms that live in the water
abiotic factors
Abiotic Factors

Certain abiotic factors affect water quality. For example:

  • pH (level of acidity/alcalinity)
  • Turbidity (non-dissolved solids)
  • Phosphates
  • Temperature
  • Dissolved Oxygen
what to know
What to know:

For each factor, make sure you know :

  • The definition/description of the factor
  • The acceptable level
  • The consequences if the level is not acceptable
  • How to measure the level
le ph
Le pH
  • pH (p 102-103)

The pH scale measures whether a substance is acidic or basic (alcaline)

pH 1 : a strong acid (like car battery acid or your stomach acid)

pH 6 : a weak acid (like milk)

pH 7 : neutral – neither acidic nor basic (like distilled water)

pH 8 : a weak base (like baking soda or human blood)

pH 14 : a strong base (like oven cleaner or drain cleaner)

le ph1
Le pH

Normal level in freshwater systems in Newfoundland and Labrador – about pH 6-6,5

Acceptable level for most aquatic organisms - between pH 5 - 8.5 (depending on the organism)


If the water is too acidic, some organisms can’t tolerate the conditions (do you remember range of tolerance from Grade 7?)

We measure pH with a probe, or with a

universal indicator (such as in pH paper)

that changes colour to show the pH

according to a scale


2. Turbidity

If there are too many non-dissolved solids in suspension, the water is turbid.

Normal level: little or no non-dissolved solids


  • Unpleasant appearance
  • Animals (such as fish) may not be able to see their food or predators
  • Prevents the Sun’s rays from penetrating the water, so plants can’t photosynthesize
    • The plants don’t produce enough food for themselves
    • The plants don’t produce enough O2 for aquatic organisms

How to measure :

a. Relative scale

Use a subjective scale from 1 to 5 - 1 is very clear and 5 is very turbid/opaque

b. Secchi disc

Use a black and white disk and lower it into the water. The deeper the disk can be seen, the more clear (less turbid) the water.

Secchi disc→

ecchi disc being lowered into the water →


3. Phosphates

Phosphates are a chemical compound that always include the elements phosphorus and oxygen. They are found in fertilizers and some detergents. They are an essential nutrient for plants.

Normal level

Less than 0.1 mg/L (0.1 ppm)

les phosphates
Les phosphates


But, if levels are too high, phosphates can cause an overproduction of plants, algae in particular (this is called eutrophication)

Eutrophication – overgrowth of algae:

This causes turbidity (look back at the consequences of that). As well, bacteria feeding on all the decomposing algae consume lots of dissolved oxygen, so other oragnisms don’t have enough


4. Temperature

Can you remember the definition from last year?

Normal temperature of freshwater systems in Newfoundland and Labrador is between 0ºC and 18ºC


  • If the water is too hot or too cold, it will be outside of the range of tolerance of many organisms and they won’t survive
  • Of course, if the water is too cold, it will freeze!

Water temperature is, unsurprisingly, measured with a thermometer

dissolved oxygen
Dissolved oxygen

4. Dissolved oxygen

Dissolved oxygen refers to the quantity of oxygen in solution in water. It can be affected by abiotic and biotic factors.

Abiotic factors that affect dissolved oxygen:

  • Water temperature– the warmer the water, less dissolved oxygen (why?)
  • Turbulence – more turbulence, more dissolved oxygen
dissolved oxygen1
Dissolved oxygen

Biotic factors that affect dissolved oxygen

  • Aquatic plants produce oxygen – some of it stays in the water
  • Other organisms consume oxygen (fish, invertebrates, bacteria, etc.)
  • If there are lots of biodegradable materials in the water, decomposers will multiply and will consume lots of oxygen, so there may not be enough for other organisms (for example, too much phosphate…too many algae…lots of dying algae/plants…turbidity…lots of decomposers…less dissolved oxygen!)
dissolved oxygen2
Dissolved oxygen

Normal level

Most aquatic organisms need at least 5 mg/L (= 5 ppm) of dissolved oxygen to survive – especially larger organisms such as fish.


If there isn’t enough dissolved oxygen in the water, organisms can’t survive.

How to measure :

  • We mix a prepared chemical solution with a water sample. The shade of blue it turns is compared to a number of standard samples (i.e. if it is the same shade as the bottle with 2 mg/L, that is the level of dissolved oxygen).
biological indicators
Biological indicators

1. Biological indicators

Some organisms can tell us something about water quality. Note: biological indicators don’t AFFECT water quality; they are AFFECTED BY the water quality.

  • Some organisms can tolerate less clean and more polluted water, where there isn’t lots of dissolved oxygen
  • Other organisms can’t survive unless the water quality is excellent – they have a narrow range of tolerance for dissolved oxygen, temperature or other pollutants

Normally, the wider the variety of insects and other aquatic invertebrates, the cleaner the water.

If we only find leeches, worms and midge larvae, the water is probably not of good quality.